How can I change the shipping price at checkout with an observer or plugin?

Question

The shipping method may be flatrate or FedEx or freeshipping, but I want to override this price with $1. How can I do this?

Answer 1

1. Register Plugin

app/code/Acme/StackExchange/etc/di.xml

<?xml version="1.0" encoding="UTF-8"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Quote\Model\Quote\Address\Rate">
        <plugin name="Acme_StackExchange_Plugin_Quote_Model_Quote_Address_RatePlugin"
                type="Acme\StackExchange\Plugin\Quote\Model\Quote\Address\RatePlugin" sortOrder="10"/>
    </type>
</config>

2. Implement Plugin logic

app/code/Acme/StackExchange/Plugin/Quote/Model/Quote/Address/RatePlugin.php

<?php
declare(strict_types=1);

namespace Acme\StackExchange\Plugin\Quote\Model\Quote\Address;

use Magento\Quote\Model\Quote\Address\Rate;
use Magento\Quote\Model\Quote\Address\RateResult\AbstractResult;
use Magento\Quote\Model\Quote\Address\RateResult\Method;

class RatePlugin
{
    public function afterImportShippingRate(
        Rate $subject,
        Rate $result,
        AbstractResult $rate
    ): Rate {
        if ($rate instanceof Method) {
            // some logic to checks
            // set custom price to method
            $result->setPrice(1);
        }

        return $result;
    }
}